1. Field of the Invention
The present invention relates to a motor driving apparatus that supplies, during power running, electric energy stored in a capacitor so as to reduce current supplied from a power source.
2. Description of the Related Art
A motor driving apparatus comprising a converter for converting an AC voltage from an AC power source to a DC voltage, an inverter for converting the converted DC voltage back to an AC voltage, a capacitor connected in parallel with a DC link section between the converter and the inverter, and a charge/discharge control circuit for controlling charge to and discharge from the capacitor is disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-141440.
The relationship between the motor speed, the current of the DC link section, and the power of the DC link section of the motor driving apparatus will be described with reference to the FIG. 10.
The motor speed is shown in the first graph in FIG. 10. In this example shown in FIG. 10, the motor is first accelerated and then decelerated.
The current of the DC link section is shown in the second graph in FIG. 10. During acceleration of the motor, current flows through the DC link section as shown in the graph. When input current to the inverter exceeds a threshold I, a switch of a discharge circuit turns on and electric energy stored in the capacitor is discharged rapidly. As a result, output current from the converter decreases rapidly.
When the motor starts decelerating, regenerative electric energy flows into the capacitor. The surplus regenerative electric energy beyond the capacitance of the capacitor is processed through power source regeneration or resistance discharge.
The power of the DC link section is shown in the third graph. During acceleration of the motor, power is supplied from the power source until output level P is reached and, after output level P is exceeded and the switch of the discharge circuit is turned on, power is supplied from the capacitor. Accordingly, electric energy in the capacitor is discharged rapidly after the switch of the discharge circuit is turned on.
When the motor starts decelerating, regenerative electric energy flows into the capacitor and the surplus regenerative electric energy beyond the capacitance is processed through power source regeneration or resistance discharge.
In the above motor driving apparatus, during power running of the motor, electric energy stored in the capacitor is not controlled or control is made so as to prevent a prescribed current amount from being exceeded after the switch of the discharge circuit is turned on, so electric energy stored in the capacitor is discharged rapidly, making it difficult to supply electric energy from the capacitor to the inverter for a sufficient period of time.
Accordingly, a large-capacity capacitor for storing much electric energy must be used to reduce supply current from the power source to a desired level. In addition, supply current from the power source is interrupted or reduced according to the amount of electric energy supplied from the capacitor to the inverter, so it is difficult to reduce the amount of current from the power source to a desirable level.